Card or picture mounts and mounting tabs for pictures have been around for well over a hundred years.
Hutton U.S. Pat. No. 739,315 is an early patent showing triangular mounts in a slotted frame or mount for receiving a picture or the like.
Dexter U.S. Pat. No. 1,410,555, Riley U.S. Pat. No. 1,999,423, Milmoe U.S. Pat. No. 1,493,159 and Engel U.S. Pat. No. 2,085,187 show typical corner mounts and supports of various configurations.